A number of patents and publications are cited herein in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Each of these references is incorporated herein by reference in its entirety into the present disclosure, to the same extent as if each individual reference was specifically and individually indicated to be incorporated by reference.
Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise,” and variations such as “comprises” and “comprising,” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.
It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a pharmaceutical carrier” includes mixtures of two or more such carriers, and the like.
Ranges are often expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment.
This disclosure includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
Nerve Injury
As yet, there are no effective treatments for nerve injuries including spinal cord injuries (SCI), stroke, and peripheral nerve injuries. The inventors have identified a novel signalling mechanism—the retinoid signalling pathway—that can be stimulated in models of nerve injury leading to axonal outgrowth and functional recovery. See, for example, Maden and Corcoran, 2000. This pathway is activated by retinoic acid (RA) binding to the retinoic acid receptor (RAR) that acts in the nucleus to drive the synthesis of RNA and hence produces proteins for axonal outgrowth. The inventors have shown that the RARβ32 subtype is specifically involved in this process.
Retinoid Signalling and Neurite Outgrowth
There are at least three causes for the lack of axonal outgrowth of central nervous system (CNS) neurons after spinal cord injury. First: the presence of growth inhibiting molecules, including Nogo-A, myelin-associated glycoprotein (MAG) and oligodendrocyte myelin glycoprotein (Omgp) (see, e.g., He and Koprivica, 2004). Second: insufficiency of growth-promoting factors, which are well-known for their ability to promote neurite outgrowth in vitro and to induce some axonal outgrowth when administered to injured cord (see, e.g., Schnell et al., 1994; Lu et al., 2004). Third: the lack of an appropriate ‘growth programme’ by damaged neurones (see, e.g., Kwon and Tetzlaff, 2001). One factor that can induce such a growth programme is RA signalling (see, e.g., Quinn and De Boni, 1991). This is mediated by RARs and retinoid X receptors (RXRs), both of which have three subtypes (α, β, and γ and various isoforms) (see, e.g., Bastien and Rochette-Egly, 2004). Transcription occurs when RA binds to an RAR/RXR heterodimer which then binds to retinoic acid response elements (RAREs) located in the regulatory regions of target genes (see, e.g., Bastien and Rochette-Egly, 2004).
RARβ2 Signalling Mediates Neurite Outgrowth
Retinoid signalling is important for the development of the embryo. When the nervous system is deprived of RA during development, neurite outgrowth fails, for example, in the RA deficient embryo (see, e.g., Maden et al., 1996; White et al., 1998). By using a panel of RAR agonists, the inventors have shown that RARβ signalling is required for retinoid mediated neurite outgrowth of neurons, whereas RARα or RARγ signalling has no effect (see, e.g., Corcoran et al., 2000). More specifically it is the activation of RARβ2 that mediates this effect (see, e.g., Corcoran et al., 2000) and this is auto-regulated by its ligand (see, e.g., Leid et al., 1992). Activation of RARβ2 by retinoids results in neurite outgrowth of cultured embryonic dorsal root ganglia (DRG), spinal cord, and adult DRG (see, e.g., Corcoran et al., 2000; Corcoran and Maden, 1999; So et al., 2006; Corcoran et al., 2002). When RARβ2 is transduced into cultured adult rodent spinal cord explants, which do not normally express this receptor, neurite outgrowth occurs (see, e.g., Corcoran et al., 2002).
RARβ2 Signalling Mediates Axonal Outgrowth
A test of the importance of RARβ signalling in axonal outgrowth comes from gene-deleted RARβ null mice. In a peripheral nerve crush model, axonal outgrowth is impeded compared to normal mice which express RARβ2 in their DRG neurons (see, e.g., Corcoran and Maden, 1999; So et al., 2006).
Furthermore, it can be demonstrated that RARβ2 expression is essential for axonal outgrowth in vivo by overexpressing it in models of spinal cord injury. In rodents, models of avulsion (where the axons of the peripheral sensory axons are damaged leading to forelimb paralysis), the overexpression of RARβ2 into the neurons of the injured DRG leads to axonal outgrowth across the dorsal root entry zone (DREZ) and back into the spinal cord leading to functional recovery (see, e.g., Wong et al., 2006).
Another model of spinal cord lesion is one that severs the corticospinal tract (CST). The cell bodies of these CST neurons are located in the brain. The CST forms the major descending pathway in the dorsal columns of the spinal cord and their damage results in functional impairments of some motor tasks. The CST lesion can be achieved by the crush of the spinal cord at the level of C4 in rodents. This results in loss of function of the forelimbs. Recently, it has been demonstrated that overexpression of RARβ2 by lentiviral vectors in adult CST neurons results in outgrowth of CST axons and functional recovery of the forelimb (see, e.g., Yip et al., 2006).
The inventors have now shown that RARβ agonists are likely to be useful in the treatment of nerve injury. RARβ agonists initiate axonal outgrowth in models of nerve injury and functional recovery occurs. Studies demonstrating these findings are described in more detail in the Examples below.    Lund et al., 2005, describes certain 4,4′-biphenylcarboxlyic acid compounds that allegedly have RARβ2 agonist activity.    Kikuchi et al., 2000, describes certain tetrahydro-tetramethyl-2-quinoxaline compounds that allegedly have RARα agonist activity.    Yoshimura et al., 2000, describes certain benzofuranyl-pyrrole and benzothiophenyl-pyrrole compounds that allegedly have RARα agonist activity.    Seino et al., 2004, describes the use of an RARα agonist, ER-38925, in the prevention of actuate and chronic allograft rejection.    Tagami et al., 2000a, Tagami et al., 2000b, and Tagami et al., 2002 all describe certain compounds allegedly exhibiting retinoic acid receptor agonism.    Kikuchi et al., 2001 describes certain compound allegedly having the activity of retinoic acid.    Tsuda et al., 1999, describes certain compounds which allegedly are useful in the treatment of pollakiuria and urinary incontinence.    Cai et al., 2003 and Cai et al., 2005 describe certain compounds which allegedly are activators of caspases and inducers of apoptosis.    Olsson et al., 2009, describes certain compounds that allegedly have activity at RARβ2 receptors.